Back before Brawl, I mained Roy. Problem was that facing off against Marths I'd always lose unless the guy made a stupid mistake. So yeah, that always sucked.

The mod is obviously addressing the issue, but what are they changing about him?

Assasin keeps track of Project M news? WHAT?!?!?!

Anyway, about Roy, here is the release video, a bit of gameplay showing his new moveset and changes, with some epic background music playing.Here is his character page on the website. A bit more expansive and explanatory than the video, and features a couple pictures along with the text.

They Upgraded his speed and his aerial game.
Roy is now usable in the air and not completely grounded.

He's just-plain more powerful too. Uncharged Neutral-B actually has good knockback. I'm pretty sure his Down-smash has more knockback too, and is both harder to punish and hits behind 'em quicker. Since Roy's only reliable KO move in Melee was F-smash, those two buffs alone are a huge boost.

Oh, and three new moves. His Side-B (Double-Edged Dance) is the same thing but the fourth strikes for each'a the directions are all new and vastly improved. Forward has retarded range, Up is simply improved, Down is probably improved but I can't really tell from just that one trailer. Also, unlike Marth, he now gets a backwards-striking forth hit for Side-B. Not just t' hit opponents behind 'em either, IIRC he can combo into it. Pretty sure the third downward-hit for Side-B can send 'em just a bit behind ya.

His Neutral-aerial is now a single hit spinning slash with huge range, and doesn't require a close-range sweetspot like most of his other moves. It's presumably a great move; clearly better than his old N-air. His F-tilt is now a really long ranged stab. Between N-air and F-tilt, Roy now has two really great ranged attacks so he can still be a threat when forced into a defensive position by an advancing foe. They're also tough to punish from what I can tell, you can act outta the moves very quickly. One'a Roy's biggest flaws was how easy he could be punished.

EDIT: Oh, and his B-air is deadly now. Has really high knockback. His D-air meteor smash is (presumably) a lot easier t' hit too, so he actually has better aerial KO options than Marth.

Ken Combo can be meteor canceled early/teched on a stage's side like Yoshis Melee, and doesn't work at higher percents when DI'd. King Combo is situational since you need t' be close to the stage... and can be DI'd, naturally. Whereas Roy can F-air -> Neutral B which clearly looks t' be easier to set up than Ken, or raw B-air for what I'm perceivin' as more KO power than Dolphin Slash.